Question

Im trying to take a text file and use only the first 30 lines of it in python.
this is what I wrote:

text = open("myText.txt")
lines = myText.readlines(30)
print lines

for some reason I get more then 150 lines when I print?
What am I doing wrong?

Answer 1

The sizehint argument for readlines isn’t what you think it is (bytes, not lines).

If you really want to use readlines, try text.readlines()[:30] instead.

Do note that this is inefficient for large files as it first creates a list containing the whole file before returning a slice of it.

A straight-forward solution would be to use readline within a loop (as shown in mac’s answer).

To handle files of various sizes (more or less than 30), Andrew’s answer provides a robust solution using itertools.islice(). To achieve similar results without itertools, consider:

output = [line for _, line in zip(range(30), open("yourfile.txt", "r"))]

or as a generator expression (Python >2.4):

output = (line for _, line in zip(range(30), open("yourfile.txt", "r")))
for line in output:
    # do something with line.